Abyssal sequestration of nuclear waste and other types of hazardous waste

ABSTRACT

A system and method of disposing nuclear waste and other hazardous waste includes means for, and the steps of, blending a waste stream, which includes either a radioactive waste or a hazardous waste (or both), with a liquid and, optionally, a solid material to produce a dense fluid and pumping the dense fluid into a tubing string of an injection boring. The dense fluid then exits a perforation in a casing of the injection boring and enters a fracture in a rock strata, where it continues to propagate downward until it reaches an immobilization point. The dense fluid may be a slurry formed by a metal and a cross-linked polymer gel or hydrated clay slurry. The metal can be one that has a melting temperature less than the temperature at the bottom of the injection boring. The solid material could also be other nuclear waste or a radionuclide.

CROSS-REFERENCE TO CO-PENDING APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/344,243, filed Nov. 4, 2016, now U.S. Pat. No. 9,741,460 B2, which isa continuation of U.S. patent application Ser. No. 14/942,643, filedNov. 16, 2015, now U.S. Pat. No. 9,700,922 B2, which is a continuationof U.S. patent application Ser. No. 14/129,504 filed Dec. 26, 2013, nowU.S. Pat. No. 9,190,181 B2, which was published on Aug. 7, 2014 underPublication No. US2014/0221722 A1, which is United States National Phaseof PCT Patent Application No. US2012/045084 filed on Jun. 29, 2012,which was published on Jan. 3, 2013 under Publication No. WO 2013/003796A1, which claims priority to U.S. Provisional Patent Application No.61/502,557 filed Jun. 29, 2011, which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Nuclear reactors generate 19 percent of the electricity in the U.S., andthis process generates high-level radioactive waste in the form ofuranium oxide or mixed oxide fuels. Approximately 1000 m³ (6200 bbl) ofhigh-level waste is produced each year from commercial reactors in theU.S., and additional material is generated by military operations.Europe is also heavily invested in nuclear power (e.g. more thanthree-fourths of the electricity in France is generated by nuclearreactors), and other countries worldwide have started to aggressivelypursue nuclear energy to power their growing economies.

As a result, the current rate of nuclear waste generation isapproximately 10,000 m³/yr, and the amount of radioactive waste beinggenerated worldwide is expected to increase significantly. Yet, thereare no safe, reliable ways to dispose of nuclear waste on site, that is,at the source of the waste's generation. This waste includes but is notlimited to spent nuclear fuel from nuclear reactors, high-level wastefrom the reprocessing of spent nuclear fuel, transuranic waste mainlyfrom defense programs, and uranium mill tailings from the mining andmilling of uranium ore. High-level nuclear waste is currently stored atthe reactor where it was generated. The only serious options fordisposal being considered are to place the waste in low permeabilitygeologic formations, like tight rock or clay. The current approach fordisposal of radioactive waste is not without problems. Congress hasmandated a 10,000-year period of isolation, but it is difficult toguarantee that waste at the shallow depths of current repositories willremain isolated from the biosphere, or human intervention, for even afraction of this time.

Yucca Mountain, a 300-m-deep facility near Las Vegas, is the only U.S.option for high-level waste disposal. This facility has been scrutinizedfor 20 years, and even after a $50 B expenditure the earliest it couldopen is 2017. Considerable political opposition by Congress, the stateof Nevada and others may delay opening even further. For example,Congress did not provide any funding for development of the site in the2011 federal budget. Significant uncertainty exists about thefeasibility of waste placed at a depth of 300 m remaining isolated fromthe biosphere for 10,000 years, and this uncertainty is the basis formuch of the opposition to Yucca Mountain. Even if Yucca Mountain doesopen, all its capacity has been allocated and options for additionalcapacity are being considered.

The politics involved in finding permanent disposal sites is, at best,difficult and, at worst, intractable. Because the waste remainsradioactive for a very long time, no one wants this waste travelingthrough their “backyard” on its way to a permanent disposal site or intheir “backyard” as the disposal site. As politicians and the publiccontinue to debate the issue, the waste remains temporarily stored onsite in ways that are arguably far less safe than any proposed permanentdisposal solution. For example, nuclear reactors temporarily store thewaste on site in water pools. The devastating earthquake and tsunami innortheast Japan, which knocked out power sources and cooling systems atTokyo Electric Power Co's Fukushima Daiichi plant, demonstrates howtenuous and potentially dangerous this storage practice really is.

Therefore, a need exits for a safe, reliable method of disposing nuclearwaste on site and one that could achieve the 10,000 year period ofisolation required by Congress and sought by other countries.

SUMMARY OF THE INVENTION

A system and method according to this invention involves storing nuclearwaste or hazardous waste in hydraulic fractures driven by gravity, aprocess referred to herein as “gravity fracturing.” For the purposes ofthis disclosure, nuclear or radioactive waste is considered a hazardouswaste although in the environmental industry radioactive waste is oftennot labeled as “hazardous waste.” The method creates a dense fluidcontaining waste, introduces the dense fluid into a fracture, andextends the fracture downward until it becomes long enough to propagateindependently. The fracture will continue to propagate downward to greatdepth, permanently isolating the waste.

Storing solid wastes by mixing the wastes with fluids and injecting theminto hydraulic fractures is a well-known technology in the petroleumindustry. Nuclear waste was injected into hydraulic fractures at OakRidge in the 1960s. The essence of the invention differs fromconventional hydraulic fracturing techniques in that it uses fracturingfluid heavier than the surrounding rock. This difference is fundamentalbecause it allows hydraulic fractures to propagate downward (rather thanhorizontally) and carry wastes by gravity instead of by pumping.

More specifically, the method of disposing nuclear waste and otherhazardous waste includes the steps of blending the waste with water orother fluid and a weighting material to make a dense fluid or slurry ofa predetermined density, temperature and viscosity; and injecting thedense fluid or slurry—at a predetermined pressure and/or rate into awell so that the fluid or slurry enters the strata at a predetermineddepth and continues to travel downward through the strata until thefluid or slurry, becomes immobilized. Prior to the blending step, thewaste, if in solid form, may be ground into particles of a predeterminedsize. The pressurized blended mixture cracks and dilates the rockstructure, which is preferably a stable, low permeability rock structuresuch as many igneous and metamorphic rocks as well as some sedimentaryrocks. (Initially, propping the fracture is avoided). Because the densefluid has a density greater than that of the rock, the fluid or slurryhas an absolute tendency to travel downward by gravity (until thedensity relationship changes or other mechanics arrest the downwardtravel) and remain far below the earth's surface. The dense fluid mayinclude water, oil, gel or any fluid suitable for providing the requiredviscosity and density.

The well is preferably drilled at and on the site which generates thenuclear waste or other hazardous waste, thereby eliminating the need totransport the waste off-site and to the disposal site. The well includesa work string or tubing for receiving the blended fluid, waste andweighting material; a packer; and a cemented steel casing withperforations located at or about the predetermined depth. Thepredetermined depth is preferably in a range of about 10,000 to 30,000feet (about 3,000 to 9,000 meters) but it can be shallower or deeperdepending upon rock properties and drilling limitations. The weightingmaterial may be other nuclear waste (including, for example,radionuclides such as uranium), other hazardous waste or a metal such asbismuth, lead, or iron in order to add weight to the primary waste whichis being disposed. Metals or alloys that are in liquid phase at thetemperature and pressure encountered in the subsurface are particularlysuitable as a weighting material.

The work string may be pulled for routine cleaning or replacement. Theblender used to blend the water, waste and weighting material ispreferably shielded, as is the pumping unit (e.g., a pumping truck) usedto pump the mixture at pressure into the well.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a preferred embodiment of a methodaccording to this invention for disposing of nuclear waste and otherhazardous waste. A dense fluid is introduced into a fracture (see FIG.2) and continues to propagate downward by way of gravity (see FIG. 3).

FIG. 2 is a well suitable for use in the practice of the method ofFIG. 1. Rather than injecting the fluid sidewise into the well,alternate embodiments of the well could inject the fluid on other ways,including at the bottom.

FIG. 3 illustrates the dense fluid of FIG. 1 as the dense fluid isintroduced into a fracture and extends the fracture downward until itbecomes long enough to propagate independently. The fracture continuesto propagate downward to great depth, permanently isolating the waste.Although not illustrated, the dense fluid may propagate downward andthen curve in a horizontal direction creating a sub-horizontal storagespace.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hydraulic fractures are created when the pressure in a fluid-filledcrack causes the material at the crack tip to fail. The fractureadvances and fluid flows forward to fill the newly created space.Hydraulic fractures are commonly created by using a pump to inject fluidinto a well, but this is by no means the only occurrence. Geologicexamples are well known in which hydraulic fractures grow upward throughthe Earth's crust because the fractures are filled with liquid lighterthan their enveloping rock. A dike filled with magma that propagatesupward to feed a volcanic eruption is one example of a hydraulicfracture propagating by gravity.

A system and method according to this invention involves propagatinghydraulic fractures downward by filling the fractures with dense fluidcontaining waste. Propagation occurs when the pressure in the fracturecreates a stress intensity that exceeds the toughness or strength of therock. Referring to FIGS. 1 to 3, an open borehole is created and filledwith the dense fluid until the pressure at the bottom is sufficient tocreate a fracture (FIG. 3 at “a”). A similar fracturing process occursduring overbalanced drilling when the mud weight is too great and causescirculation to be lost by initiating a fracture and causing it to growaway from the borehole. Fluid will flow into the fracture and the levelof fluid in the well will drop (FIG. 3 at “b”). However, the fracture isexpected to advance faster than the rate of drop of fluid level in thewell, so the overall height from the tip of the fracture to the top ofthe fluid column in the well lengthens. This increases the drivingpressure and furthers downward propagation as the fluid in the wellboredrains by gravity into the fracture (FIG. 3 at “c”).

The vertical span of the fracture continuously increases, causing thepressure at the bottom of the fracture to increase and ensuringcontinued downward propagation, even after all the liquid has drainedfrom the well into the fracture (FIG. 3 at “d”). The pressuredistribution causes the lower part of the fracture to bulge open and theupper part to pinch shut. A residual coating of fluid will be leftbehind when the fracture closes, and this will diminish the volume offluid in the fracture. Eventually the original fluid will be spread as athin coating on the fracture wall, extending from the bottom of theborehole to great depth. In the case of slurry, the fracture may bepropped if the liquid leaks off into the rock.

The process is repeated by putting additional fluid into the well. Thiswill create a new fracture that will follow the path of the earlier one(FIG. 3 at “e”). The additional fluid reaches an even greater depth thanthe original batch. The maximum depth that can be reached by densefluids is unclear, but it could exceed tens of kilometers.

A method of disposing nuclear waste and other hazardous waste practicedaccording to this invention, therefore, effectively removes the wastefrom exposure to human activities at a time scale relevant to bothsocietal actions and the half-lives of many hazardous radionuclides. Themethod includes the steps of blending the waste with materials suitablefor creating a dense fluid or slurry which has a predetermined densityand viscosity; and injecting the dense fluid at a predetermined pressureor rate into a well so that the dense fluid enters the strata at apredetermined depth and continues to travel downward through the stratauntil its flow stops, for example, because the solid-to-liquid ratio istoo high to allow flow. Propagation may also stop when a sufficientamount of the dense fluid or fluid/slurry has been spread as a film orresidue over the upper closed portion of the fracture.

Oil, gel or any fluid suitable for providing the required viscosity anddensity may be used Weighting material adds density to the primary wastewhich may be other types of nuclear waste, other hazardous waste or ametal such as, but not limited to, bismuth, lead, iron, copper, or lowmelting point metals or alloys (e.g., mercury, woods metal, indalloy 15,gallium) that could mix with and possibly dissolve or amalgamatehigh-level waste material. The low-melting-point alloys are a liquidunder the expected pressure and temperature conditions at the bottom ofthe injection well. Solid compounds such as metals used for weightingmaterial may be mixed with a high-shear-strength liquid, includingpolymer gels that may be crosslinked, or inorganic gels that may formedby hydrating clay minerals, to create a dense slurry. Prior to theblending step, the waste, if in solid form, may be ground to apredetermined size.

The pressurized dense fluid creates a vertical fracture or crack in therock structure. The dense fluid enters the crack and serves to prop therock structure. The rock structure is preferably a stable, lowpermeability rock formation, of the kind that nuclear reactors aretypically built over and upon. Because of the weighting material, thedensity of the dense fluid is greater than that of the rock and thiscauses an absolute tendency for the fluid to travel downward until itbecomes immobilized. If the density of the dense fluid is exactly equalto that of the rock, the dense fluid may be unable to overcome the rockfracture toughness. This is required for fracture propagation, hence thedensity should be somewhat higher to ensure the fracture growth. Howmuch higher depends upon the fracture toughness magnitude, fluidproperties, and other effects standard in industrial hydraulicfracturing.

In general terms, the density of rock increases as depth increases.Therefore, once the fracture propagates, a point can be reached wherethe density of the dense fluid becomes the same as the density of therock, thereby limiting any further propagation downward. Eventually, thefracture becomes sub-horizontal and the dense fluid fills the fracturehorizontally. This is similar to geological sills and does not hamperthe proposed technology as the horizontal part of the growing fracturealso allows for safe waste storage. Fracture toughness also increaseswith depth because it increases with such factors as temperature,pressure and size of the fracture. However, the effect of fracturetoughness can be overcome by pressurizing the fracture.

For example, and just by way of example the immobilization point mayoccur at about 2,000 to 50,000 feet (about 600 to 15,000 meters) belowthe dense fluid's initial entry point into the strata. (The depth can begreater and is mostly constrained by drilling and pumping limitations.)The dense fluid can be monitored by using conventional tracer means tosee whether any movement or migration has occurred upward relative tothe perforations in the well casing, or it can be monitored usingmicroseismics means to evaluate downward migration below the bottom ofthe region accessible to the well casing.

The well is preferably drilled at and on the site which generates thenuclear waste or other hazardous waste, thereby eliminating the need totransport the waste off-site and to the disposal site. The well alsoeliminates the need for temporary storage means on site because thewaste can be transported directly to the well for immediate permanentdisposal. As shown in FIG. 2, the well includes a work string or tubingfor receiving the blended water, waste and weighting material; a packer;and a cement casing with perforations located at or about thepredetermined depth. The predetermined depth is preferably in a range ofabout 10,000 to 30,000 feet (about 3,000 to 9,000 meters). The workstring may be pulled for routine cleaning or replacement. The blenderused to blend the water, waste and weighting material is preferablyshielded, as is the pump truck used to pump the dense fluid at pressureinto the well (see FIG. 1).

Preferred embodiments of a system and method for abyssal sequestrationof nuclear waste and other types of hazardous waste have been describedand illustrated, but not all possible embodiments. The inventive systemand method itself is defined and limited by the following claims.

What is claimed is:
 1. A system for abyssal sequestration of nuclear waste and other types of hazardous waste, the system comprising: a gravity fracture filled with a fluid having at least one waste selected from the group consisting of a radioactive waste and a hazardous waste, the fluid being denser than a rock formation into which the fluid is to be disposed so as to cause the rock formation to gravity fracture, the fluid propagating downward in the gravity fracture as the gravity fracture propagates downward.
 2. A system according to claim 1 wherein the fluid has a density of at least 3.0 g/cm³.
 3. A system according to claim 1 wherein the fluid is a slurry.
 4. A system according to claim 3 further comprising the slurry including a solid material which is blended with the at least one waste.
 5. A system according to claim 4 wherein the solid material is a metal.
 6. A system according to claim 5 wherein the metal is selected from the group consisting of bismuth, iron, lead, and copper.
 7. A system according to claim 3 wherein the solid material contains one or more radionuclides.
 8. A system according to claim 3 wherein a liquid component of the slurry is a metal having a melting temperature less than a temperature at a bottom end of an injection boring from which the fluid exits into the rock formation.
 9. A system according to claim 8 wherein the metal is selected from the group consisting of mercury, woods metal, indalloy 15, and gallium.
 10. A system according to claim 1 further comprising the fluid including a liquid.
 11. A system according to claim 10 wherein the liquid includes at least a portion thereof selected from the group consisting of a cross-linked polymer gel and a hydrated clay slurry.
 12. A system according to claim 10 wherein the liquid is a metal having a melting temperature less than a temperature at a bottom end of an injection boring from which the fluid exits into the rock formation.
 13. A system according to claim 10 wherein the liquid is a metal selected from the group consisting of mercury, woods metal, indalloy 15, and gallium.
 14. A system according to claim 1 further comprising the fluid including a solid material.
 15. A system according to claim 14 wherein the solid material is a metal.
 16. A system according to claim 15 wherein the metal is selected from the group consisting of bismuth, iron, lead, and copper.
 17. A system according to claim 14 wherein the solid material contains one or more radionuclides. 